Since I don't own an iPhone, your assumption is incorrect. My spouse and many other non-technical people I know do own iPhones. And it is very common for them to use apps rather than their browser to access web content.
I call shenanigans. Practically every "content" site out there has its own app, and loudly proclaims its existence whenever you visit its site with a compatible device. It's simply not possible that "it never occurred" to you. You can obviously choose not to install a Google+ app if you like. But I don't believe for a minute that you are surprised at the need for it.